The hs_err head is:

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/opt/jprt/T/P1/194642.bpb/s/hotspot/src/share/vm/runtime/synchronizer.cpp:660), pid=4720, tid=140053811422976
# assert(temp->is_neutral()) failed: invariant
#
# JRE version: Java(TM) SE Runtime Environment (8.0_40) (build 1.8.0_40-internal-fastdebug-20141222194642.bpb.hs-dev-b00)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (25.60-b01-fastdebug mixed mode linux-amd64 compressed oops)
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
#

--------------- T H R E A D ---------------

Current thread (0x00007f6100315000): JavaThread "forceEarlyReturn001aTestThread_0" [_thread_in_vm, id=4816, stack(0x00007f60d19de000,0x00007f60d1adf000)]

Stack: [0x00007f60d19de000,0x00007f60d1adf000], sp=0x00007f60d1adce80, free space=1019k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0xffb80a] VMError::report_and_die()+0x2ca
V [libjvm.so+0x72b424] report_vm_error(char const*, int, char const*, char const*)+0x84
V [libjvm.so+0xf27f93] ObjectSynchronizer::FastHashCode(Thread*, oop)+0x233
V [libjvm.so+0xab730b] JVM_IHashCode+0x10b
j java.lang.Object.hashCode()I+0
j java.lang.Object.toString()Ljava/lang/String;+23
j java.lang.String.valueOf(Ljava/lang/Object;)Ljava/lang/String;+10
j java.lang.StringBuilder.append(Ljava/lang/Object;)Ljava/lang/StringBuilder;+2
j nsk.share.jpda.ForceEarlyReturnTestThread.executeMethod(Ljava/lang/String;)V+822
j nsk.share.jpda.ForceEarlyReturnTestThread.run()V+82
v ~StubRoutines::call_stub
V [libjvm.so+0x9c00a4] JavaCalls::call_helper(JavaValue*, methodHandle*, JavaCallArguments*, Thread*)+0x1684
V [libjvm.so+0x9c29bb] JavaCalls::call_virtual(JavaValue*, KlassHandle, Symbol*, Symbol*, JavaCallArguments*, Thread*)+0x7bb
V [libjvm.so+0x9c34c8] JavaCalls::call_virtual(JavaValue*, Handle, KlassHandle, Symbol*, Symbol*, Thread*)+0xc8
V [libjvm.so+0xac19bf] thread_entry(JavaThread*, Thread*)+0xcf
V [libjvm.so+0xf7c581] JavaThread::thread_main_inner()+0x221
V [libjvm.so+0xf7c8bd] JavaThread::run()+0x2ed
V [libjvm.so+0xd838f8] java_start(Thread*)+0x108
C [libpthread.so.0+0x7df3] start_thread+0xc3

Java frames: (J=compiled Java code, j=interpreted, Vv=VM code)
j java.lang.Object.hashCode()I+0
j java.lang.Object.toString()Ljava/lang/String;+23
j java.lang.String.valueOf(Ljava/lang/Object;)Ljava/lang/String;+10
j java.lang.StringBuilder.append(Ljava/lang/Object;)Ljava/lang/StringBuilder;+2
j nsk.share.jpda.ForceEarlyReturnTestThread.executeMethod(Ljava/lang/String;)V+822
j nsk.share.jpda.ForceEarlyReturnTestThread.run()V+82
v ~StubRoutines::call_stub